NADPH-cytochrome P450 reductase (P450 reductase) is a microsomal flavoprotein that transfers electrons from NADPH to microsomal cytochrome P450. It is expressed in liver and many extrahepatic tissues and is an obligatory, often rate-limiting component of microsomal P450-linked carcinogen activation and drug and alcohol metabolism reactions. While the influence of endogenous regulators on individual P450 enzymes and their genes have been studied extensively, much less is understood about the regulation of P450 reductase by endogenous hormonal factors. Studies carried out using the rat model have established that normal, physiological levels of thyroid hormone are required for full expression of hepatic P450 reductase mRNA, protein and enzyme activity, as well as P450 reductase-dependent xenobiotic metabolism. Moreover, while enhanced gene transcription was found to contribute to the thyroid hormone-dependent expression of P450 reductase mRNA, post-transcriptional mechanisms are key to the regulation of P450 reductase protein and activity. The studies described in this new grant application are designed to elucidate the post-transcriptional regulatory mechanisms by which thyroid hormone modulates P450 reductase expression under conditions of hyperthyroidism and hyperthyroidism, and to evaluate the implications of this modulation for P450 reductase-catalyzed quinone and carcinogen activation via redox cycling. The major goals of this proposal are (a) to elucidate the post-transcriptional mechanisms through which thyroid hormone regulates liver P450 reductase mRNA expression, (b) to characterize the role of thyroid hormone in the translational and post-translational control of P450 reductase protein and activity, and (c) to evaluate the effects of thyroid hormone-dependent alterations in cellular P450 reductase activity on the metabolism and redox cycling of quinones, catechols and related carcinogens, and to elucidate the impact on the cell's adaptive response to oxidative stress. The detailed understanding of the multiple actions of thyroid hormone on P450 reductase expression and foreign chemical metabolism provided by these studies will help elucidate some of the underlying mechanisms through which hormonal environment can influence xenobiotic-induced redox cycling, and may ultimately help to identify individuals who are more susceptible to oxidative stress as a consequence of alterations in thyroid hormone status.